Astronomers studying gas clouds in the famous Whirlpool Galaxy
have found important clues supporting a theory that seeks to
explain how the spectacular spiral arms of galaxies can persist
for billions of years. The astronomers applied techniques used
to study similar gas clouds in our own Milky Way to those in
the spiral arms of a neighbor galaxy for the first time, and
their results bolster a theory first proposed in 1964.

The Whirlpool Galaxy, about 31 million light-years distant,
is a beautiful spiral in the constellation Canes Venatici.
Also known as M51, it is seen nearly face-on from Earth and
is familiar to amateur astronomers and has been featured in
countless posters, books and magazine articles.

"This galaxy made a great target for our study of spiral arms
and how star formation works along them," said Eva Schinnerer,
of the
National Radio Astronomy Observatory in Socorro, NM.
"It was ideal for us because it's one of the closest face-on
spirals in the sky," she added.

Schinnerer worked with Axel Weiss of the Institute for
Millimeter Radio Astronomy (IRAM) in Spain, Susanne Aalto of
the Onsala Space Observatory in Sweden, and Nick Scoville of
Caltech. The astronomers presented their findings to the
American Astronomical Society's meeting in Denver, Colorado.

The scientists analyzed radio emission from Carbon Monoxide
(CO) molecules in giant gas clouds along M51's spiral arms.
Using telescopes at Caltech's Owens Valley Radio Observatory
and the 30-meter radio telescope of IRAM, they were able to
determine the temperatures and amounts of turbulence within
the clouds. Their results provide strong support for a
theory that "density waves" explain how spiral arms can
persist in a galaxy without winding themselves so tightly
that, in effect, they disappear.

The density-wave theory, proposed by Frank Shu and C.C. Lin
in 1964, says that a galaxy's spiral pattern is a wave of
higher density, or compression, that revolves around the galaxy
at a speed different from that of the galaxy's gas and stars.
Schinnerer and her colleagues studied a region in one of M51's
spiral arms that presumably has just overtaken and passed
through the density wave.

Their data indicate that gas on the trailing edge of the
spiral arm, which has most recently passed through the density
wave, is both warmer and more turbulent than gas in the
forward edge of the arm, which would have passed through
the density wave longer ago.

"This is what we would expect from the density-wave
theory," Schinnerer said. "The gas that passed through the
density wave earlier has had time to cool and lose the
turbulence caused by the passage," she added.

"Our results show, for the first time, how the density wave
operates on a cloud-cloud scale, and how it promotes and
prevents star formation in spiral arms," Aalto said.

The next step, the scientists say, is to look at other spiral
galaxies to see if a similar pattern is present. That will
have to wait, Schinnerer said, because the radio emission from
CO molecules that provides the information on temperature and
turbulence is very faint.

"When the
Atacama Large Millimeter Array (ALMA) comes on line,
it will have the ability to extend this type of study to other
galaxies. We look forward to using ALMA to test the
density-wave model more thoroughly," Schinnerer said. ALMA is
a millimeter-wave observatory that will use 64, 12-meter-diameter
dish antennas on the Atacama Desert of northern Chile. Now
under construction, ALMA will provide astronomers with an
unprecedented capability to study the Universe at millimeter
wavelengths.

The Whirlpool Galaxy was discovered by the French comet-hunter
Charles Messier on October 13, 1773. He included it as object
number 51 in his now-famous catalog of astronomical objects that,
in a small telescope, might be mistaken for a comet. In 1845,
the British astronomer Lord Rosse discovered the spiral
structure in the galaxy. For amateur astronomers using telescopes
in dark-sky locations, M51 is a showpiece object.